In order to determine the composition of measuring fluids, such as pure liquids, liquid mixtures, emulsions, or suspensions, analyzers are often used in process measurement technology or in environmental analytics. An analyzer generally comprises a sensing element which is designed to generate a measurement signal dependent upon at least one analytical measurand, as well as an electronic evaluation unit which from the measurement signal determines a measured value representing the current value of the at least one analytical measurand in the measuring fluid. The electronic measuring unit can be integrated at least partially into a control and evaluation unit of the analyzer, said control and evaluation unit comprising display and input means, by means of which a user can enter and retrieve information.
The analytical measurand can, for example, be a measurand dependent upon the concentration of one or more ingredients of the measuring fluid. Such measurands are, for example, the concentration or activity of certain ion types, such as nitrate, phosphate, or ammonium in water samples, or even sum parameters of water analytics, such as COD (chemical oxygen demand), TOC (total organic carbon), TNb (total nitrogen), or TP (total phosphorus), which depend upon the concentration of several different ingredients in the sample. The measuring fluid can be taken automatically by means of an automatic sample preparation unit at a sampling point from a process or from a body of water to be examined and held available in a so-called sample receiving vessel. In order to determine a measured value of the measurand, an analyzer is often designed to take a sample of the liquid contained in the sample receiving vessel and to preprocess it for example, by adding reagents. The ingredient(s) to be determined can even be present in the measuring fluid in an undissolved state, which frequently requires dissolution to be carried out before measurement of the measurand by means of the sensing element.
By means of the sensing element, the analyzer determines one or more measured values of a parameter of the possibly preprocessed sample, said parameter correlating with the measurand to be determined and dependent upon the concentration or activity of one or more ingredients of the measuring fluid. The preprocessing of the sample often comprises the addition of one or more reagents to the sample. The reagents are generally selected such that the preprocessing results in a chemical reaction involving the ingredient(s), upon the concentration or activity of which the measurand to be determined depends. The reaction product can be detected by means of the sensing element. The parameter detected by the sensing element can correlate especially with the concentration of the reaction product, especially with an intensity of a coloration of the preprocessing liquid sample, said coloration being caused by the reaction product. Oftentimes, the sensing element is designed as a photometric or spectrometric sensing element that detects an absorption or extinction of the preprocessed sample at one or more wavelengths as a parameter correlating with the measurand and generates one or more electrical measurement signals representing this absorption or extinction. Based upon the measurement signals of the sensing element, the control and evaluation unit of the analyzer can derive measured values of the measurand for example, by using a stored correlation rule, possibly determined by a calibration, in the form of a table or a function.
Such analyzers are known, for example, from DE 10 22 822 A1, DE 10 2009 029 305 A1, or DE 10 2011 075 762 A1.
In DE 10 2011 088 235 A1, for example, an analyzer for the automatic determination of a measurand dependent upon a concentration of an ingredient of a measuring fluid is described, which comprises a sample receiving vessel as well as one or more liquid tanks for one or more reagents, a cuvette for receiving a reaction mixture generated by mixing a liquid sample taken from the sample receiving vessel with one or more reagents, and a sensing element for providing one or more measurement signals correlating with the measurand. The analyzer comprises a control and evaluation unit that is designed to control the analyzer and to determine the measurand based upon the measurement signals provided by the sensing element. The analyzer also comprises a delivery and metering unit controlled by the control and evaluation unit, to deliver and meter the liquid sample and liquids from the liquid tanks into the cuvette.
The fill level of the sample receiving vessel can be monitored in such analyzers by means of a suitable level sensor, in order to ensure that only in the presence of a sufficient quantity of measuring fluid is a new measurement cycle of the analyzer started. If this monitoring, however, fails, or if the liquid lines that connect the sample receiving vessel to the sensing element are clogged or have leaks, it can happen that the sample taken from the sample receiving vessel for preprocessing has a volume that is less than the sample volume specified by the control and evaluation unit or that no measuring fluid at all arrives in the cuvette. Since the liquid lines in conventional analyzers have relatively small cross-sections in order to manage with the smallest sample and reagent volumes possible, gas bubbles occurring in the liquid lines can also result in the sample volumes being too small. However, if the sample volume actually delivered does not match the sample volume specified by the control and evaluation unit, or if no measuring fluid arrives in the cuvette, incorrect measurements can result.
Even if non-plausible measured values due to incorrect measurements are detected, an evaluation of the source of the error based only upon the current measured values is problematic.
Accordingly, there remains a need for further contributions in this area of technology to enable a means of ensuring a desired sample volume.